This invention relates to a process for forming combustible liquid and gaseous light hydrocarbons from carbonaceous materials. More particularly, it relates to improvements in the known steam-iron process for gasification of materials such as coal.
The so called steam-iron process for producing methane rich gases is essentially a two step procedure wherein, in a first step, steam is contacted with iron and particulate carbonaceous solids at a temperature and pressures such that light hydrocarbons and FeO are produced (gas producing stage). The oxidized iron is then transferred to a zone wherein, as a second step, it is reduced to form iron and recycled (iron reduction stage).
While this process is per se old, prior art attempts at commercialization directed to forming methane or mixtures of methane and other low molecular weight combustible liquids or gases as a substitute for natural gas have been fraught with significant problems. One problem is that of agglomeration of the carbonaceous materials in the gas producing stage. When heated to the temperature required to effect the gasification reaction, carbonaceous material particles tend to stick together to form large agglomerates or "klinkers" and to produce other tarry deposits which clog the apparatus employed, seriously reduce the theoretical efficiency of the reactions, and effectively prohibit the design of a smooth running continuous process. Coals having a high net hydrogen to carbon ratio are particularly susceptible to this behavior. Bituminous coal particles, for example, when heated to a temperature on the order of 600.degree. K., form a sticky liquid surface layer which acts as a binder for adjacent similar particles. This phenomenon results in debilitating decreases in reaction rates because of the decrease in surface area of the solids reacting and the clogging problems.
Another problem inherent in the prior art steam-iron process is that of reducing the iron oxide for recycling. Obviously, the cost of recycling must be very low in order to minimize the cost of the gas produced. One proposed iron-oxide reduction process involves contacting the iron oxides with solid carbonaceous materials. However, this method of reduction is relatively slow due to the notoriously poor kinetics of reactions dependent on solid-solid contact. Another known iron reduction process involves utilizing carbon monoxide or a mixture of carbon monoxide and hydrogen as a reducing agent. Hydrogen, of course, is quite expensive. Furthermore, in the case of both hydrogen and carbon monoxide, large amounts of gas are required because of the low equilibrium conversion characteristics of reduction using these gases. Low reaction rates necessarily result in large and expensive reactors.
Other disadvantages of the prior art processes include unacceptably low breakdowns of steam, thereby requiring means to condense large volumes of steam with the attendant generation of large quantities of relatively useless low temperature heat. Furthermore, because of the inefficiencies inherent in the gas producing stage, the prior art processes frequently produced significant quantities of carbon monoxide together with the low molecular weight hydrocarbons. This occurred primarily in prior art processes which utilized insufficient quantities or surface area of metallic iron such that they were kinetically controlled.